They made the discovery in a project to map the distribution of hydrogen gas across the galaxy, New Scientist will report this weekend.

Most of the Milky Way is obscured by interstellar dust, but hydrogen emits radiowaves that pass through the dust clouds, allowing detection by radio telescope.

"We see [the arm] over a huge area of sky," said lead astronomer Dr Naomi McClure-Griffiths.

She speculated the arm is a long gaseous tendril that was once joined up with another spiral limb but became detached.

Astronomers are amazed that the feature had been overlooked.

"I was absolutely flabbergasted; it was quite clearly seen in some of the previous surveys but it was never pointed out or given a name," Dr Tom Dame at the Harvard-Smithsonian Center for Astrophysics in Massachusetts told New Scientist.